1. Technical Field
This application relates to a detecting liquid and solid substances through the use of very low power applied electromagnetic fields, and more particularly to an apparatus and method suitable for both detecting the same through both Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR).
2. Background
Various “spectroscopic” techniques are used to measure a variety of different atomic and molecular properties (concentration, amount, type, molecular structure, and much more) through an instrument that gives a signal response as a function of frequency (or energy); i.e., an instrument that gives a spectrum. Examples of such techniques include Raman absorption, and Mossbauer spectroscopy, nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and nuclear quadrupole resonance (NQR).
NMR, MRI and NQR all rely on the same general principle, that is, a nuclear resonance triggered by the use of radio frequency (RF) pulses. However, NQR differs from NMR and MRI as it does not require an external magnetic field; the nuclear spin states arise from the interaction between the nuclear charge density and the electric field gradient (EFG) at the nucleus, caused by neighboring charges.
To produce nuclear magnetic resonance (NMR) signals, large and powerful magnets are typically needed. Moreover, the stronger the magnetic field, the stronger the received signal and the more detailed the obtained information.
NQR is a solid-state RF spectroscopic technique able to detect compounds with quadrupolar nuclei, i.e., with spin quantum number I>½ (“spin ½ particles”). Around half of the elements of the periodic table has this property. As opposed to NMR and MRI, no external magnetic field is required for NQR, allowing for portable instruments. However, since the nuclei are not aligned, the signals are very weak.
For more details of the theory behind NMR and NQR detection, see Gudmundson, E., “Signal Processing for Spectroscopic Applications”, Department of Information Technology, Uppsala University, SE-751 05, Uppsala, Sweden, © 2010.
Multinuclear NMR/MRI spectroscopy is the name given to the study of NMR active nuclei of elements other than hydrogen 1 (proton) or carbon 13. A wide range of elements with NMR frequencies ranging from silver (18.62 MHz) to Phosphorus (161.98 MHz) can be utilized to detect chemical compounds of interest. Hydrogen is the most frequently imaged nucleus because it is present in biological tissues in great abundance. Any nucleus with a net nuclear spin can potentially be imaged or detected by MRI/NMR. Sodium 23 and Phosphorus 31 are naturally abundant in the human body and could be imaged or detected directly.
As mentioned above, NMR requires large static magnetic fields, typically ranging from 500 Gauss to 20,000 Gauss to create Zeeman splitting of the original nuclei state. A high power RF pulse is used to excite nuclei from the lower state to the higher energy state. The excited nuclei fall back to the ground state causing free induction decay which is observed as a weak decaying pulse with oscillations at the Larmor frequency.
In nuclear quadrupole resonance (NQR) the splitting of the original nuclei state is caused by the electric fields from the surrounding electron cloud of the atom. Many current NQR systems utilize a high power RF pulse which is used to excite nuclei from the lower state to the higher energy state. Similar to NMR, the excited nuclei fall back to the ground state causing free induction decay which is detected as a decaying RF pulse.
Some have proposed the use of Superconducting Quantum Interference Devices (SQUIDs) as a sensitive detector of magnetic flux for both NMR and NQR spectroscopy. See for example, Augustine, M. P. et al “SQUID Detected NMR and NQR”, in Solid State Nuclear Magnetic Resonance, Vol 11 (1998) pp. 139-156. SQUIDs introduce a number of complexities, not the least of which is the need for cryogenic cooling to operate them; the resulting SQUID structures are different for the NQR and NMR detection modalities.